In returning to the UK permanently, is one required to tell any government departments?

The mere fact of going through immigration, I would have thought, shows that one is back, but one could be just visiting of course.

Are there penalties for failing to tell?

You are not required to tell anyone by law per se although you should do all the normal things one would do when moving to a new UK address, e.g. update the electoral roll, update your driving licence, registered with a local GP, etc..

A British Citizen doesn't have to 'go through immigration' so no-one knows you are back, so telling HMRC isn't a bad idea, if you do go through DWP I believe that they will offer to tell other government departments you are back.

Thanks, BritInParis.

If I remember correctly, when I arrived at Manchester Airport a few months ago, I had to show my British passport to an official in front of the queue of British (EU) passport holders, then went through a machine which scanned my thumb and took my photograph. What was that about?

Just proving that you matched the passport, you might only have been visiting for a week's holiday.

Ah, okay. Thanks.

Everyone who arrives at the UK border will need to pass through immigration. The only exception is those arriving from within the Common Travel Area. The Home Office will now log your entries and exits from the UK but their system isn't joined to, say, HMRC or the DWP. You can inform HMRC that you are leaving or have left the UK using Form P85 but there is no equivalent form for arrivals (if you haven't this already then I would do so now to ensure your tax and NI record is up to date - you may receive a rebate). If you need to then you simply start interacting again with the relevant government department.

So the home office knows when I enter the UK on my British passport but is it really smart enough to tie it together with my Canadian passport when I leave the UK!!?

Probably not.